It’s a business decision.
From the “Sharing Your Content and Information” section of the terms page http://www.facebook.com/legal/terms
“You own all of the content and information you post on Facebook, and you can control how it is shared through your privacy and application settings. In addition:
1. For content that is covered by intellectual property rights, like photos and videos (“IP content”), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (“IP License”). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.”
You own your content and information, but then you grant them permission to do almost anything with your work.
If you’re trying to make money with your photography, you’ve put yourself in competition with YOURSELF!